portlist: update some internals to use append-style APIs

In prep for reducing garbage, being able to reuse memory.  So far this
doesn't actually reuse much. This is just changing signatures around.

But some improvement in any case:

    bradfitz@tsdev:~/src/tailscale.com$ ~/go/bin/benchstat before after
    name       old time/op    new time/op    delta
    GetList-8    11.8ms ± 9%     9.9ms ± 3%  -15.98%  (p=0.000 n=10+10)

    name       old alloc/op   new alloc/op   delta
    GetList-8    99.5kB ± 2%    91.9kB ± 0%   -7.62%  (p=0.000 n=9+9)

    name       old allocs/op  new allocs/op  delta
    GetList-8     3.05k ± 1%     2.93k ± 0%   -3.83%  (p=0.000 n=8+9)

More later, once parsers can reuse strings from previous parses.

Updates #5958

Change-Id: I76cd5048246dd24d11c4e263d8bb8041747fb2b0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

portlist/netstat.go

@@ -55,7 +55,7 @@ type nothing struct{}
 // formats that we can parse without special detection logic.
 // Unfortunately, options to filter by proto or state are non-portable,
 // so we'll filter for ourselves.
-func parsePortsNetstat(output string) List {
+func appendParsePortsNetstat(base []Port, output string) []Port {
 	m := map[Port]nothing{}
 	lines := strings.Split(string(output), "\n")
 
@@ -131,13 +131,18 @@ func parsePortsNetstat(output string) List {
 		lastline = ""
 	}
 
-	l := []Port{}
+	ret := base
 	for p := range m {
-		l = append(l, p)
+		ret = append(ret, p)
 	}
-	sort.Slice(l, func(i, j int) bool {
-		return (&l[i]).lessThan(&l[j])
+
+	// Only sort the part we appended. It's up to the caller to sort the whole
+	// thing if they'd like. In practice the caller's base will have len 0,
+	// though, so the whole thing will be sorted.
+	toSort := ret[len(base):]
+	sort.Slice(toSort, func(i, j int) bool {
+		return (&toSort[i]).lessThan(&toSort[j])
 	})
 
-	return l
+	return ret
 }